1. Field of Invention
The present invention relates generally to an inverter circuit using voltage converter having multiple layers of piezoelectric ceramic. More specifically, the present invention relates to a Cold Cathode Fluorescent Lamp (CCFL) inverter circuit incorporating a multilayer piezoelectric transformer that uses the longitudinal resonant vibration mode for step-up voltage conversion.
2. Description of the Prior Art
Wire wound-type electromagnetic transformers have been used for generating high voltage in internal power circuits of devices such as televisions or fluorescent lamp ballasts. Such electromagnetic transformers take the form of a conductor wound onto a core made of a magnetic substance. Because a large number of turns of the conductor are required to realize a high transformation ratio, electromagnetic transformers that are effective, yet at the same time compact and slim in shape are extremely difficult to produce. Furthermore, in view of high frequency applications, the electromagnetic transformer has many disadvantages involving magnetic material of the electromagnetic transformer, such as sharp increase in hysteresis loss, eddy-current loss and conductor skin-effect loss. Those losses limit the practical frequency range of magnetic transformers to not above 500 kHz.
To remedy this and many other problems of the wire-wound transformer, piezoelectric ceramic transformers (or PTs) utilizing the piezoelectric effect have been provided in the prior art. In contrast to electromagnetic transformers, PTs have a sharp frequency characteristic of the output voltage to input voltage ratio, which has a peak at the resonant frequency. This resonant frequency depends on the material constants and thickness of materials of construction of the transformer including the piezoelectric ceramics and electrodes. Furthermore PTs have a number of advantages over general electromagnetic transformers. The size of PTs can be made much smaller than electromagnetic transformers of comparable transformation ratio, PTs can be made nonflammable, and produce no electromagnetically induced noise.
The ceramic body employed in PTs takes various forms and configurations, including rings, flat slabs and the like. Typical examples of a prior PTs are illustrated in FIGS. 1 and 2. This type of PT is commonly referred to as a "Rosen-type" piezoelectric transformer. The basic Rosen-type piezoelectric transformer was disclosed in U.S. Pat. No. 2,830,274 and numerous variations of this basic apparatus are well known in the prior art. The typical Rosen-type PT comprises a flat ceramic slab 20 appreciably longer than it is wide and substantially wider than it is thick. In the case of FIG. 1, the piezoelectric body 20 is in the form of a flat slab that is considerably wider than it is thick, and having greater length than width.
As shown in FIG. 1, a piezoelectric body 20 is employed having some portions polarized differently from others. A substantial portion of the slab 20, the generator portion 22 to the right of the center of the slab is polarized longitudinally, and has a high impedance in the direction of polarization. The remainder of the slab, the vibrator portion 21 is polarized transversely to the plane of the slab's face (in the thickness direction) and has a low impedance in the direction of polarization. In this case the vibrator portion 21 of the slab is actually divided into two portions. The first portion 24 of the vibrator portion 21 is polarized transversely in one direction, and the second portion 26 of the vibrator portion 21 is also polarized transversely but in the direction opposite to that of the polarization in the first portion 24 of the vibrator portion 21.
In order that electrical voltages may be related to mechanical stress in the slab 20, electrodes are provided. If desired, there may be a common electrode 28, shown as grounded. For the primary connection and for relating voltages at opposite faces of the low impedance vibrator portion 21 of the slab 20, there is an electrode 30 opposite the common electrode 28. For relating voltages to stresses generated in the longitudinal direction in the high impedance generator portion 22 of the slab 20, there is a secondary or high-voltage electrode 35 on the end of the slab for cooperating with the common electrode 28. The electrode 35 is shown as connected to a terminal 34 of an output load 36 grounded at its opposite end.
In the arrangement illustrated in FIG. 1, a voltage applied between the electrodes 28 and 30 of the low impedance vibrator portion 21 is stepped up to a higher voltage between the electrodes 28 and 35 in the high impedance generator portion for supplying the load 36 at a much higher voltage than that applied between the electrodes 28 and 30. The applied voltage causes a deformation of the slab thickness which results in proportionate changes in the x-y and y-z surface areas. More specifically, the Rosen PT is operated by applying alternating voltage to the drive electrodes 28 and 30, respectively. A longitudinal vibration is thereby excited in the low impedance vibrator portion 21 in the transverse effect mode (d31 mode). The transverse effect mode vibration in the low impedance vibrator portion 21 in turn excites a vibration in the high impedance generator portion 22 in a longitudinal effect longitudinal vibration mode (g33 mode). As the result, high voltage output is obtained between electrode 28 and 35. On the other hand, for obtaining output of step-down voltage, as appreciated, the high impedance portion 22 undergoing longitudinal effect mode vibration may be used as the input and the low impedance portion 21 subjected to transverse effect mode vibration as the output.
An inherent problem of such prior PTs is that they have relatively low power transmission capacity. This disadvantage of prior PTs relates to the fact that little or no mechanical advantage is realized between the vibrator portion 21 of the device and the driver portion 22 of the device. Because the driver and vibrator portions each is intrinsically a part of the same electroactive member, the transmission of energy between portions is limited to Poisson coupling. This inherently restricts the mechanical energy transmission capability of the device, which, in turn, inherently restricts the electrical power handling capacity of such devices.
Additionally, even under resonant conditions, because the piezoelectric voltage transmission function of Rosen-type PTs is accomplished by proportionate changes in the x-y and y-z surface areas (or, in certain embodiments, changes in the x-y and x'-y' surface areas) of the piezoelectric member, which changes are of relatively low magnitude, the power handling capacity of prior circuits using such piezoelectric transformers is inherently low. Because the power transmission capacity of such prior PTs is so low, it has become common in the prior art to combine several such transformers together into a multi-layer "stack" in order to achieve a greater power transmission capacity than would be achievable using one such prior transformer alone. This, of course, increases both the size and the manufacturing cost of the transformer.
In addition, with the typical Rosen transformer, it is generally necessary to alternately apply positive and negative voltages across opposing faces of the vibrator portion 21 of the member in order to "push" and "pull", respectively, the member into the desired shape. Even under resonant conditions, prior electrical circuits that incorporate such prior PTs are relatively inefficient, because the energy required during the first half-cycle of operation to "push" the piezoelectric member into a first shape is largely lost (i.e. by generating heat) during the "pull" half-cycle of operation. This heat generation corresponds to a lowering of efficiency of the circuit, an increased fire hazard, and/or a reduction in component and circuit reliability. In order to reduce the temperature of such heat generating circuits, the circuit components (typically including switching transistors and other components, as well as the transformer itself) are oversized, which reduces the number of applications in which the circuit can be utilized, and which also increases the cost/price of the circuit.
Also generally known are PTs polarized and vibrating in the thickness direction (i.e., vibrations are parallel to the direction of polarization of the layers). Illustrative of such thickness mode vibration PTs is the device of U.S. Pat. No. 5,118,982 to Inoue shown in FIG. 3. A thickness mode vibration PT typically comprises a low impedance portion 11 and a high impedance portion 12 stacked on each other. The low impedance portion 11 and the high impedance portion 12 of the thickness mode PT typically comprises a series of laminate layers of ceramic alternating with electrode layers. Each portion is composed of at least two electrode layers and at least one piezoelectric material layer. Each of the piezoelectric ceramic layers of the low impedance portion 11 and the ceramic layer of the high impedance portion 12 are polarized in the thickness direction (perpendicular to the plane of the interface between the ceramic layers). Every alternate electrode layer in each portion 11 or 12 may be connected to each other and to selected external terminals.
The thickness mode PT of FIG. 3 comprises a low impedance vibrator portion 11 including a plurality of piezoelectric layers 211 through 214 and a high impedance vibrator portion 12 including a piezoelectric layer 222, each of the layers being integrally laminated, and caused to vibrate in thickness-extensional mode. The low impedance portion 11 has a laminated structure which comprises multi-layered piezoelectric layers 211 through 214 each being interposed between electrodes including the top surface electrode layer 201 and internal electrode layers 231 through 234. The high impedance portion 12 is constructed of the bottom electrode layer 202, an internal electrode layer 234 and a single piezoelectric layer 122 interposed between both electrode layers 202 and 234. Polarization in each piezoelectric layer is, as indicated by arrows, in the direction of thickness, respectively. In the low impedance portion 11, alternating piezoelectric layers are polarized in opposite directions to each other. The polarization in the high impedance portion 12 is also in the direction of thickness. The PT has a common electrode 234 to which one terminal 16 of each portion is connected. The total thickness of the PT of FIG. 3 is restricted to a half wavelength (lambda/2) or one full wavelength (lambda) of the drive frequency.
When an alternating voltage is applied to the electrode layers across the ceramic layer of the vibrator portion 11, a vibration is excited in the ceramic parallel to the direction of the polarization of the layers in the longitudinal vibration mode (d33 mode). This vibration of the low impedance portion 11 excites a vibration (g33 mode) in the high impedance portion 12. As the high impedance portion 12 vibrates, the g33 mode deformation of the high impedance portion 12 generates an electrical voltage across the electrodes of the high impedance portion 12. When operating the PT in the thickness-extensional mode with a resonance of lambda/2 mode (both end free fundamental mode) or lambda mode (both end-free secondary mode), the PT may operate in a typical frequency range of 1-10 MHz.
Referring now to FIG. 4: It is characteristic of PTs is that they preferably vibrate in a resonant mode predominantly along one plane or direction (i.e., radial or longitudinal planes, and thickness or longitudinal directions). A problem occurs with PTs when the ratio between the longitudinal or radial dimension is close to the thickness dimension. When the ratio between the height H and the radius R of the PT are close to unity, then radial or longitudinal mode resonant frequency and thickness mode resonant frequency are also close to each other. When the resonant frequencies are very close to each other, then the vibrations interfere with each other. This leads to aberrant vibrational modes that reduce the efficiency of these PTs.
Another problem with prior thickness mode PTs is that the thickness mode resonant frequency is too high for some applications.
Another problem with prior thickness mode and Rosen PTs is that they do not have a sufficient power transmission capacity for some applications.
Another problem with prior thickness mode and Rosen PTs is that the addition of layers makes the PT profile (height) too high to be placed within miniaturized circuits.
Another problem with prior thickness mode PTs is that the addition of layers makes the thickness dimension to close to the longitudinal or radial dimensions.
Another problem with prior thickness mode PTs is that the addition of layers to the PT does not significantly raise the power density of such devices and may increase capacitive and dielectric losses.
Accordingly, it would be desirable to provide a piezoelectric transformer design that has a higher power transmission capacity than similarly sized prior piezoelectric transformers.
It would also be desirable to provide a piezoelectric transformer that is smaller than prior piezoelectric transformers that have similar power transmission capacities.
It would also be desirable to provide a piezoelectric transformer that is a low profile (height to length ratio) as compared to prior piezoelectric transformers that have similar power transmission capacities.
It would also be desirable to provide a piezoelectric transformer with a significant difference between dimensions in thickness and longitudinal dimensions thereby reducing aberrant vibrational modes
It would also be desirable to provide a piezoelectric transformer in which the "driver" portion of the device and the "driven" portion of the device are not the same electro-active element.
It would also be desirable to provide a piezoelectric transformer that develops a substantial mechanical advantage between the driver portion of the device and the driven portion of the device.
It would also be desirable to provide an inverter circuit incorporating a piezoelectric transformer of the character described.